In order to design an optical system so as to have a specified degree of correction for chromatic aberration, it is necessary to use an appropriate combination of optical materials for the refractive elements of the system. A technique is described in U.S. Pat. No. 5,020,889 for identifying appropriate combinations of optical materials to use in designing optical systems that are to have a specified degree of color correction.
Techniques are described in U.S. Pat. Nos. 4,958,919 and 5,033,831 for selecting combinations of liquids and glasses for use as the lens elements of optical systems that are to be well-corrected for chromatic aberration.
Unless an appropriate combination of optical materials is used for the refractive elements of an optical system, it would not be possible to achieve a specified degree of correction for chromatic aberration for the system. However, the selection of an appropriate combination of optical materials is not a sufficient condition for achieving the specified degree of correction for chromatic aberration. In addition to using an appropriate combination of optical materials, the designer must also determine an appropriate design form for the system--i.e., an appropriate set of geometrical relationships for the refractive elements that comprise the system.
Until recently, techniques as described in the above-cited references for selecting appropriate combinations of optical materials for use in designing optical systems that are corrected for chromatic aberration over specified wavelength ranges had generally not been applied to the design of lens systems incorporating a liquid optical element in the infrared wavelength band, because there has been practically a complete absence of refractive index measurements at infrared wavelengths for optically useful liquids.
Recent collaboration between researchers at the Lockheed Palo Alto Research Laboratories in Palo Alto, Calif. and the Vavilov State Optical Institute in St. Petersburg, Russia has resulted in the measurement of refractive indices for a number of liquids at infrared wavelengths. The data obtained thereby has made possible the use of liquids to design optical systems that are well-corrected for chromatic aberration over a broad wavelength band through the near infrared region of the electromagnetic spectrum.